1. Field of the Invention
The present invention concerns a process and apparatus for treating a gaseous effluent from a Claus plant or a gas containing hydrogen sulphide and sulphur dioxide.
In particular, it concerns the treatment of effluents from Claus plants from hydrodesulphurization and catalytic cracking units. It also concerns the purification treatment of natural gas.
2. Description of the Prior Art
The prior art is illustrated by French patent application FR-A-2 411 802.
The Claus process is widely used to recover elemental sulphur from gaseous feeds containing hydrogen sulphide (H.sub.2 S). However, the fumes emitted from these Claus type plants, even after several catalytic stages, contain non negligible amounts of acid gases. Those effluents (tail gases) from Claus plants must, therefore, be treated to eliminate the majority of toxic compounds so as to satisfy anti-pollution regulations. These regulations are becoming more and more strict and existing technology must be constantly improved.
As an example, about 95% by weight of the sulphur present can be recovered from a Claus plant; treatment of this Claus plant tail gas (using a Clauspol unit, for example) can recover 99.8% by weight of the sulphur, for example, using the reaction: EQU 2H.sub.2 S+SO.sub.2.fwdarw.3 S+2H.sub.2 O
which uses a reaction medium constituted by an organic solvent and a catalyst comprising an alkaline or alkaline-earth salt of an organic acid. The reaction is generally carried out in counter-current mode in a reactor-contactor and its temperature is controlled by passing the solvent which is extracted from the lower end of the reactor by a circulating pump through a heat exchanger to encourage the highest possible degree of conversion to sulphur while avoiding the formation of solid sulphur. Sulphur is thus recovered in liquid form. While it is very effective, the process is limited by various constraints:
The thermodynamic equilibrium of the reaction is such that the reaction is never complete. Some hydrogen sulphide and sulphur dioxide remains, in equilibrium with the sulphur and water which are formed. The quantity of sulphur present in unreacted H.sub.2 S and SO.sub.2 which is found in the reaction effluent (from the Clauspol) corresponds to about 0.1% of the total sulphur in the initial feed to the Claus plant. Better conversion can be envisaged at a lower operating temperature but this temperature must be kept above the freezing point of sulphur (about 120.degree. C.) otherwise the reactor will be blocked with solid sulphur;
The presence of unseparated liquid sulphur in the reactor-contactor, which is entrained in the solvent and catalyst which circulate, and which is recycled to the reactor-contactor. Not all of the droplets of liquid sulphur are separated from the solvent and the presence of liquid sulphur irremediably causes the presence of gaseous sulphur in the effluent due to the vapour pressure of sulphur. As an example, the quantity of unrecovered sulphur which can be attributed to vapour pressure is about 0.1% by weight of the sulphur in the initial feed. PA0 In a second variation, if the organic solvent is not miscible with water, it can be cooled in the same manner as in the first variation. Cooling is preferably by heat exchange. PA0 In the category of solvents which are soluble in water, with boiling points at atmospheric pressure of more than 200.degree. C. are polyols containing 2 to 15 carbon atoms, preferably glycerol, thiodiglycol and cyclohexanedimethylethanol, acid esters containing 5 to 15 carbon atoms, more particularly trimethylpentanediol mono-isobutyrate and dimethyl adipate, glycol ethers containing 5 to 15 carbon atoms, advantageously butoxytriglycol, ethoxytriglycol, diethylene glycol butylether, ethylene glycol phenylether, terpinyl ethylene glycol monobenzyl ether, ethylene glycol butylphenylether, diethylene glycol, diethylene glycol dimethylether, diethylene glycol dibutylether, triethylene glycol, tetraethylene glycol dimethylether, propylene n-butylether, dipropylene n-butylether, tripropylene n-butylether, and polyethylene glycol with a molar mass of 200, 300, 400 or 600.